Synthesis of Nanocrystalline Boron Carbide by Direct Microwave Carbothermal Reduction of Boric Acid

Hindawi Journal of Nanomaterials Volume 2017, Article ID 3983468, 8 pages https://doi.org/10.1155/2017/3983468

Research Article Synthesis of Nanocrystalline Boron Carbide by Direct Microwave Carbothermal Reduction of Boric Acid

Rodolfo F. K. Gunnewiek, Pollyane M. Souto, and Ruth H. G. A. Kiminami

Department of Materials Engineering, Federal University of Sao˜ Carlos, Rod. Washington Luiz, km 235, 13565-905 Sao˜ Carlos, SP, Brazil Correspondence should be addressed to Rodolfo F. K. Gunnewiek; [email protected]

Received 24 January 2017; Accepted 12 March 2017; Published 27 March 2017

Academic Editor: Stefano Bellucci

Copyright © 2017 Rodolfo F. K. Gunnewiek et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

The excellent physical and chemical properties of boron carbide make it suitable for many applications. However, its synthesis requires a large amount of energy and is time-consuming. Microwave carbothermal reduction is a fast technique for producing well crystallized equiaxial boron carbide nanoparticles of about 50 nm and very few amounts of elongated nanoparticles were also synthesized. They presented an average length of 82 nm and a high aspect ratio (5.5). The total reaction time was only 20 minutes, which disfavor the growing process, leading to the synthesis of nanoparticles. Microwave-assisted synthesis leaded to producing boron-rich boron carbide. Increasing the forward power increases the boron content and enhances the efficiency of the reaction, resulting in better crystallized boron carbide.

1. Introduction (with 20%-at. of carbon) and even high carbon content boron carbide B11.4C3.6 (B3.17C), which corresponds to 24%-at. of Boron carbide is covalent carbide, which presents some of the carbon [2, 14–16]. The composition of boron carbide affects best physical and chemical properties. B4C is the third hardest its properties, since carbon-rich crystals approach the ideal materialandshowshighYoung’smodulus,highmeltingpoint ∘ crystal and have the lowest electrical conductivity, while (2450 C without decomposing), excellent chemical resis- 3 boron-rich crystals are more resistant to radiation damage. tance, very low density (2.52 g/cm ), low stiffness, excellent However, the ideal boron carbide structure, B4C, is composed thermoelectric properties, and good cross section absorption of B11C icosahedra and C-B-C chains comprising the longest of thermal neutrons. These properties make it suitable for diagonal in the rhombohedral structure [2]. numerous applications from wear resistant components and Boron carbide can be synthesized by different methods, cutting tools to high temperature thermocouples and neu- for example, magnesiothermic and aluminothermic reduc- tron absorbers [1–5]. Boron carbide can also be used as a tion, both of which are exothermic reductions of B2O3 in the precursor or reduction agent to produce boron nitride and presence of metallic magnesium or aluminum, and a carbon transition-metal diborides (TiB2,ZrB2,HfB2,andCrB2)[6– source [2, 16–18], vapor phase reaction [2], polymer precur- 10] and recently has been applied in gamma ray scintillator sors [17, 19–22], liquid phase reaction [23], and solid state and neutron detectors and as high efficiency thermoelectric reaction [15] are other alternative routes to producing boron material [11, 12]. carbide. The carbothermal reduction, a technique widely It belongs to the rhombohedral lattice R3m space group, used in industry and laboratory to produce mostly carbides, although a recent paper describes a second B4Cphasewith including boron carbide, consists of reacting an oxide (boron monoclinic structure, reopening the discussion about the oxide) and a carbon source at high temperatures, which boron carbide structure [13]. Boron carbide can be found normally takes several hours to complete [2]. in a wide range of carbon and boron compositions: from An inexpensive and efficient way to synthesize boron B11.4C (with 8%-at. of carbon) to the most well-known B4C carbide by the aforementioned technique is using boric acid 2 Journal of Nanomaterials

Multimodal applicator power Forward

Ceramic tube Stirrer

Argon inlet

Alumina boat and precursors

ermal insulator Figure 1: Schematic diagram of the microwave reaction system.

and carbon black or charcoal as starting materials [24–28]. reduction of boric acid. Microwave forward power, which The general reaction between boric acid and carbon is very influences the synthesis of boron carbide, is also evaluated for simple: first the boric acid dehydrates and is converted to both stoichiometric and hyperstoichiometric raw materials. boron anhydride, followed by the reduction to elemental boron by carbon, and finally the reaction of this species 2. Experimental Procedures with carbon, yielding boron carbide. This reaction is efficient 2 and pure boron carbide can be produced at temperatures Disordered carbon black with high surface area (263.2 m /g) ∘ above 1400 C and normally a long reaction time (1 h to 5 h) and pure boric acid (Merck, >99%) were used in this [2]. The reaction time and temperature, heating rate, and experiment. The raw materials, at a ratio of 4 : 7 mol of boric boron/carbon rate are essential parameters to control the acid/carbon (R4B), were suspended in ethanol (the boric acid dissolved completely) and stirred continuously on a hot plate final average particles size. Increasing both the reaction time ∘ and temperature enhances B4Csynthesis[25].Higherheating at 50 C for one hour to partially evaporate the solvent, which rates cause the raw materials to reach the final temperature rendered the suspension pasty. The partially dried paste was extruded into pellets and completely dried in a kiln at around rapidly, practically without undergoing an intermediate tem- ∘ perature. Higher temperature favors reactions in the vapor 50 C (this procedure was necessary to prevent the removal state and high levels of nucleation, which can result in finer oftheprecursorsbytheargonfluxduringthereaction).In particles. addition, to determine whether it is possible to synthesize Finer nanoparticles are hard to synthesize, because, high boron-content boron carbide, another precursor was besides the high nucleation rate caused by high temperature, prepared with a boric acid to carbon ratio of 8 : 7 (R8B). About the particles growing is also observed due the long reaction 2gofthepelletswasspreadonasmallalumina/mulliteboat time. This factor can be avoided by the use of microwaves, (50mminlength)andplacedinamicrowaveoven(2.45GHz, which reduces considerably the time and prevents excessive Cober, USA), whose reaction chamber was cleaned for 10 min particle growth. Hard materials such as TiC, TaC, WC, with an argon flow set at 1.0 L/min. The reaction systems were nanometric titanium carbide, and carbonitride have been designed to protect the oven and control the atmosphere, as produced by microwave irradiation [29–33]. depicted in Figure 1. The reaction system consisted in a low Nitrides and carbides are very difficult to sintering due porosity ceramic tube coated with refractory fiber (to avoid to their reduced diffusion coefficients. The use of nanos- heat loss) placed inside the oven cavity. An argon inlet was tructured boron carbide can enhance the sintering, because connected to the tube at one end and a gas collector at the the diffusion is accelerated due to the high surface energy, other. The rectangular boat crucible containing the precursor also leading to obtaining ceramics with controlled and finer wasinsertedintothetubeatthecenteroftheovencavity. microstructure (the benefits of finer microstructure in the The microwave forward power was set at 1.8 and 2.1 kW properties (e.g., mechanical, electrical, and others) are well for each composition. The samples were allowed to cool in known). The sintering step is influenced by the initial boron an argon atmosphere to prevent reoxidation, which took no carbide particle size and shape and also by additives, such more than 40 min. The cooled powders were deagglomerated as free carbon, which prevents the formation of boron oxide and ground in a mortar and sifted through a 325-mesh sieve. thin film [34] and a low melting point at the grain boundary, To determine the phase constitution, X-ray diffraction (XRD) aiding mass diffusion and also controlling grain growth [17]. patterns of the powders were recorded in a Siemens D5005 This paper describes a fast and efficient approach for diffractometer using copper K𝛼 radiation (𝜆 =1,5406A).˚ preparing nanocrystalline B4C by microwave carbothermal Crystallite size 𝑑 was calculated using Scherrer’s relation Journal of Nanomaterials 3

350 (proportion of 4 : 7 of H3BO3/C), the formation of boron carbide is evident, and the well-defined peaks in the XRD 300 pattern are very clear. The peaks correspond to rhombohedral B4C for R4B-1 and R4B-2 and are consistent with JCPDS card 250 number 35-798, although smaller peaks corresponding to unreacted orthoboric acid, H3BO3, and a band of disordered 200 carbon were detected. Moreover, the intensity of the peaks increased along with the power level, possibly indicating 150 more crystalline boron carbide and/or more effective reac-

Intensity (a.u.) Intensity tion, yielding a larger total amount of boron carbide when 100 synthesized at higher power. When the boric acid content in the precursor was doubled, the reaction could not be 50 completed, so practically only boric acid peaks and disordered bands are visible. Nevertheless, there are small peaks 0 corresponding to JCPDS card number 86-1126 (Figure 3(b)), 20 30 40 50 60 70 80 that is, boron-rich boron carbide (B13.38C1.62), suggesting the 2휃 presence of traces of this hyperstoichiometric boron carbide. Figure 2: X-ray diffractogram of disordered carbon black used in We believe the reaction could not occur with this large this work. amount of boric acid for the following reasons: (1)Afterthe rawmaterialsaremixedandthendried,theboricacid(or melted dehydrated boron oxide) forms a very thick layer on shownin(1),where𝜃 is the Bragg angle and 𝐵 is the sample the carbon particles, hindering the mass diffusion and, hence, line broadening at full-width at half maximum (FWHM), the full formation of boron carbide. This may explain the very which is the relation between the experimental FWHM and low percentage of possible high boron-content boron carbide. the instrumental correction. (2) The high content of B2O3 after dehydration reacted to free 0.9𝜆 carbon, forming carbon monoxide and dioxide and B2O2, 𝑑= . (1) 𝐵 𝜃 which consumed most of the carbon and yielded boron oxide cos (which rehydrated to form boric acid), a small amount of The microstructure of the powders was characterized by hyperstoichiometric boron carbide, and a very minor amount field emission gun scanning electron microscopy (FEG-SEM) of unreacted carbon, which decreased as the forward power (Philips XL30 FEG) at 25 kV and by transmission electron increased. microscopy (TEM) (TECNAI G2F20). Lower power leads to a lower reaction temperature. This effect is evident by comparing the peaks of the two samples; 3. Results and Discussion thatis,thepeaksofsampleR4B-1werelessintensethanthose of sample R4B-2, and the crystalline peaks increased consid- Figure 2 depicts the X-ray diffractogram of the carbon black erably in response to higher power. Sample R4B-2 showed used in this work, revealing that it is composed of two bands moreevidentpeaks,evenwhenthepeakwasnotrelevant, ∘ ∘ at 2𝜃 of around 20–30 and 40–50 , corresponding to a providing evidence of well crystallized rhombohedral B4Cat disordered material. ahigherpowerof2.1kW,whilesampleR4B-1synthesizedat Disordered carbon black showed a high dielectric loss a lower power (1.8 kW) presented only the main significant tangent, tan 𝛿, from 0.35 to 0.83 [36], which makes it a good diffraction peaks corresponding to the hexagonal planes (104) microwave absorber, interacting strongly with the microwave and (021). electric field, promoting the high heating very rapidly to Broadening of the main peak suggests very fine particles. the temperatures at which the reactions occur. Moreover, In fact, by applying Scherrer’s relation (1) to the main peak, the pelletization process ensures that a sufficiently porous plane(021),theaveragecrystallitesizecanbeinferredand,as microstructure is obtained, enabling the first gaseous product expected, this broadening of peaks is a sign of the presence ofthereaction,water,tobereleasedwhenboricacidbeginsto of nanoparticles of 50.5 nm in the R4B-1 powder and slightly ∘ ∘ dehydrate around 80 Candcontinuesupto330C[37].The larger ones of 57.1nm in the R4B-2 powder. ∘ B2O3 melts at around 450 C. Furthermore, the B2O3 obtained The rhombohedral lattice can also be described asa in this first carbon reduction step is still in close contact with hexagonallattice[38],and,inthecaseofB4C, the hexagonal 𝑐 the particles of carbon black. Thus, the entire carbothermal H axis is parallel to the longest diagonal of the rhombohedral reduction reaction took only 20 min, followed by 40 min of structure, which contains the three-atom chain (C-B-C). cooling, after which the diffractograms were recorded. Table 1 summarizes the calculated hexagonal and rhombohe- Figures 3(a) and 3(b) depict the diffractograms of pow- dral lattice parameters and the theoretical lattice parameters ders R4B 1 and 2 and R8B 1 and 2 (the notation 1 or 2 corre- of stoichiometric B4C (JCPDS card number 35-798). Many sponds to the microwave power level used in the synthesis: authors have described changes in lattice parameters as a 1.8 or 2.1 kW, resp.). These images clearly show the influence function of carbon and boron content in unitary cells, stating of the boron content and forward power on the synthesis that the lattice parameters increase when hyperstoichiomet- of boron carbide. At a low boric acid content (Figure 3(a)) ric boron carbide (boron-rich) is obtained [12, 15, 35, 39]. Our 4 Journal of Nanomaterials

Table 1: Lattice parameters of as-synthesized boron carbide at different levels of forward power.

𝑎 𝑐 𝑎 𝛼 Cell volume H H R 3 (nm) (nm) (nm) (degrees) (nm ) R4B-1 (1.8 kW) 0.5596 1.2061 0.5158 65.7053 0.2114 R4B-2 (2.1 kW) 0.5600 1.2070 0.5162 65.7081 0.2117 Stoichiometric B4C(theoretical) 0.5600 1.2086 0.5166 65.6486 0.2118

R8B-2 Intensity (a.u.) Intensity R4B-2 (a.u.) Intensity

R4B-1 R8B-1 30 40 50 60 70 80 20 30 40 50 60 70 80 2휃 2휃

B4C C B13,38C1,62 C

H3BO3 H3BO3 (a) (b)

Figure 3: XRD patterns of R4B (a) and R8B (b) powders reacted at different power levels: 1.8 kW (R4B-1 and R8B-1) and 2.1 kW (R4B-2 and R8B-2), obtained in only 20 min of reaction, showing crystalline boron carbide synthesis. experiments showed a very clear change in lattice parameters, yielded 18.51%-at. These values correspond to the chemical suggesting differences in the amounts of boron and carbon as formulas B4.91CandB5.40C,respectively.Thecarboncontent a function of microwave power and, in the R8B series, of the decreased slightly in response to increasing power input. In boron content in raw materials. Higher forward power leads thecaseofsampleR4B-2,whichwassynthesizedat2.1kW,the to enriching the lattice with boron, since the expansion of A&T fitting yielded a carbon content of 19.81%-at, while the 𝑐 lattice parameters is observed, particularly on the H axis. The G&C fitting yielded 18.13%-at, corresponding to the chemical possible explanation is the higher temperatures and higher formulas B5.05CandB5.51C, respectively. ∘ heating rates achieved under higher power levels, which At about 2𝜃 of 26 ,anamorphouspatternisvisible,which favors the reaction between boron suboxides and free carbon, corresponds to unreacted carbon and is more evident in the yielding hyperstoichiometric boron carbide [2]. diffractogram of sample R4B-1. The peak intensity ratio of Many authors [12, 15, 35, 39] have described the relation- B4C [22] can be estimated from the relation of B4Cand ship between the atomic percentage of carbon and the lattice amorphous band intensities, as indicated in the following: parameters of boron carbide. The contraction of the lattice 𝐼 parameters suggests an increase in the carbon content of the B4C 𝑅= . (2) lattice up to given values of 𝑐 and 𝑎 . Aselage and Tissot (𝐼 +𝐼 ) H H B4C C [15]andGossetandColin[35]reportedthreechangesinthe 𝑐 slope of H: linear fitted between 1.217 nm and 1.206 nm and The peak intensity ratios calculated for R4B-1 and R4B- two additional slopes, one below and the other above this 2 are 0.51 and 0.79, respectively, which means there is range. Werheit and Shalamberidze [39] reported a maximum more unreacted carbon when the precursors are subjected 𝑐 H at around 1.219 nm, which decreased to a lower (boron- to lower microwave power, 1.8 kW. However, boron carbide 𝑐 rich) and higher (carbon-rich) H value. can be synthesized at even lower power. Increasing the The carbon content (%-at) was estimated by fitting the power accelerated the heating rate due to the excellent second slope (Figure 4) between 1.217 nm and 1.206 nm to microwave absorption capacity of carbon, and therefore the the curves described by both Aselage and Tissot (A&T) and temperature reached in the reaction was higher. However, Gosset and Colin (G&C). The A&T fitting of sample R4B-1 the best condition for this reaction was not attained at lower showed a carbon content of 20.37%-at, while the G&C fitting power. The remaining free carbon seems to be beneficial for Journal of Nanomaterials 5

Table 2: Reaction parameters and results of microwave carbothermal reaction.

Sample Reaction time Power Average crystallite size Corresponding chemical (min) (kW) (nm) formula (A&T)

R4B-1 20 1.8 50.5 B4.91C

R4B-2 20 2.1 57.1 B5.05C

solid-liquid reaction. However, a detailed TEM micrograph G&C t y = −0.2261x + 1.2489 of the R4B-2 powder (Figure 6(b)) reveals the presence of not R2 = 0.9649 only the aforementioned equiaxial nanoparticles but also an 1.215 uncommon presence of elongated nanorod primary particles, withanaveragelengthof81.5nmandaspectratioof5.5. The presence of these particles is very rare. Even covering (nm) H

c A&T t the entire extension of TEM grid, the aforementioned mor- 1.210 y = −0.1565x + 1.2382 phology was found punctually. They might be synthesized in R2 = 0.9831 nonstoichiometric regions with low carbon content (boron- rich) and are often synthesized when the reactants are in the gaseous state [26]. 1.205 Table 2 summarizes the reaction parameters and results 0.130 0.150 0.170 0.190 0.210 of boron carbide nanopowders. A very short reaction time of %-at C 20 min under microwave irradiation leads to the formation A&T R4B-1 (G&C) of nanostructured B4Cofequivalentaveragesizeatboth G&C R4B-2 (A&T) microwave power levels, with the reaction efficiency and R4B-1 (A&T) R4B-2 (G&C) crystallization enhanced at higher power. The use of microwave radiation is highly favorable for 𝑐 Figure 4: Values reported in the literature for the hexagonal lattice both solid-liquid and gas-solid reactions. Vaporized boron parameter as a function of carbon content, and plot of values of oxide can be carried out by other gases, such as carbon samples R4B-1 and R4B-2 calculated by Aselage and Tissot (A&T) [15] and Gosset and Colin (G&C) [35] by second slope fitting. monoxide and argon flux, which may change the stoichiometry of the reaction and lead to an incomplete reaction, leaving behind free carbon. Higher temperatures favor the evaporation of boron oxide, according to Herth et al. [27], ∘ furthersinteringofthematerial,sinceitcanenhancethemass who calculated that its vaporization rate at 1300 C is 6.2 nm/s. diffusion at the grain boundaries [17]. This means that, in 20 minutes of reaction, most of the boron Owing to its high heating rate and efficient energy oxide will evaporate and can react with carbon particles via transfer, the microwave-induced reaction led to synthesis and solid-gas or can be carried out by others gases. It is stated yielded fine boron carbide particles. SEM images (Figures that fast microwave heating is advantageous in synthesizing 5(a) and 5(b)) show homogeneous boron carbide powders nanosized boron carbide without causing changes in the synthesized using microwave radiation. Figure 5(a) shows C/H3BO3 precursors, resulting in chemical compositions R4B-1 SEM powder morphology and Figure 5(b) shows close to the well-known rhombohedral B4C. According to the R4B-2 powder morphology after microwave-assisted Weimer et al. [24], higher heating rates combined with high carbothermal reduction in only 20 minutes of reaction. temperatures favor the formation of boron carbide clusters Samples R4B-1 and R4B-2 both presented a homogeneous via the reaction of C(s) or CO(g) with gas-phase boron oxide, morphology with average particle sizes of 75 and 80 nm, yielding primary particles smaller than 100 nm. respectively. Equivalent B4C nanopowders were synthesized As discussed early, the reaction can take place in three ∘ once by conventional heating, but the process took 5 hours steps: (1) boric acid dehydrates at temperatures above 80 C, [40]. Figures 5(a) and 5(b) also show some aggregates. transforming to boron anhydride (B2O3), (2) boron oxide is We believe this structure is formed early during the low reduced by carbon producing elementary boron, and (3)the temperature reaction step, when boron oxide probably melts, reaction between elementary boron and carbon forms boron yielding some clusters which will be reduced by carbon, carbide. formingthecitedstructure.Jungetal.[26]identifiedthe Upon completion of the first step of B2O3 formation, ∘ formation of clusters at very low temperature of about 700 C. the kinetics of microwave carbothermal reduction probably The SEM micrographs of the powders show equiaxial begins with the solid-liquid reaction of melted B2O3 with nanoparticles (Figure 6(a)). The average size of the equiaxial solid carbon, directly producing equiaxial boron carbide, particles, calculated from TEM micrographs, is 47.1 nm, little while the solid-gas reaction of carbon with evaporated B2O3 below the average crystallite size. The electron diffraction formsitdirectlyorviareductionofboronoxideandreaction pattern of R4B-2 in Figure 6(a) shows very well crystallized with the elementary boron. An intermediary reaction step B4C, confirming the XRD analysis. According to Jung etal. may occur, forming boron suboxide B2O2, which is highly [26], the equiaxial nanoparticles are synthesized mostly by volatile. This can help the gaseous reaction. Microwave 6 Journal of Nanomaterials

(a) (b)

Figure 5: SEM micrographs of (a) R4B-1 and (b) R4B-2. The nanoparticles were synthesized using microwaves as the heating source.

(a) (b)

Figure 6: TEM images of R4B-2: (a) details of the morphology of equiaxial nanoparticles and ED pattern with crystalline B4Cand(b) equiaxial and rare presence of elongated morphology.

power promotes high heating rates above the vaporization short dwell time at high temperature. However, a high boron temperature of boron oxide, and most of it goes to the vapor oxide ratio (R8B) is not effective for synthesis, because boron phase, which is reduced by carbon, forming boron suboxide oxide probably reacts with carbon, forming carbon monoxide and CO(g).Thesuboxide,stillinthevaporphase,formsboron and volatile B2O2. This reaction consumes the carbon or carbide by reacting with carbon and/or carbon monoxide. causes the boron oxide to melt around the carbon particles, On the other hand, B2O2 may be carried away by the argon hindering mass diffusion and preventing the effectiveness of flux, interfering in the efficiency of the reaction. The solid- the reaction. Moreover, the partial reduction of boron oxide gas reaction of B2O3 is more probable, according to the to volatile B2O2 occurs even at a higher carbon ratio, which calculation of Gibb’s free energy performed by Herth et al. is one of the reasons for the presence of residual unreacted [27]. It is believed that direct reduction by both solid-liquid carbon [26], which can be carried away by the argon flux. and solid-gas reactions of carbon and B2O3 occurred more Another factor is the short reaction time, which is described probably. bymanyauthors[26,28,41],aswellastemperature,whichis The efficient energy transfer favored by microwave pro- critical in the synthesis of boron carbide. cessing, especially because of excellent absorption capacity Carbothermal reduction is advantageous to synthesize of carbon black, enabled boron carbide to synthesize rapidly equiaxial boron carbide nanoparticles. The use of microwave without any additive such as magnesium or aluminum or power as a heating source proved to be efficient to synthesize even a catalyst such as cobalt. At the temperature of reaction, well crystallized nanostructured boron carbide. It avoided there is a competition between the kinetic processes of excessive loss of the boron oxide at high heating rates nucleation of new particles and growing, wherein the last is (or higher temperatures) as pointed by many authors [20, favored. The efficient energy transfer to carbon black leads 24, 26, 27], maintaining the reaction close to the planed to high heating rates and fast synthesis, which can contribute stoichiometry. 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